Foundation Engineering Problems and Hazards in Karst
Terranes

Please note: While the Maryland Geological Survey can help identify whether your home is located on rocks that are likely to develop sinkholes, we cannot offer site-specific advice. Local resources for problems related to sinkholes may be found on the Sinkhole Resources page.

Introduction

Just about any place where the land is underlain by relatively soluble bedrock,
natural waters on and below the land surface slowly dissolve that bedrock.
Dissolving is enhanced by these waters' tendency to be acidic. For example,
rain is usually acidic because it contains dissolved carbon dioxide (CO2) from
the atmosphere. It often becomes more acidic as it soaks into the ground and
picks up more CO2 from the soil. Such a landscape in which the bedrock is shaped,
or sculpted, by dissolution is referred to as karst. The most common type of
bedrock in karst terranes is carbonate rock, which includes limestone (calcium
carbonate), dolostone (calcium-magnesium carbonate), and marble (calcium carbonate).
There are a few other kinds of rock (e.g., gypsum, which is composed of calcium
sulfate) that can be involved in karst, but in Maryland karst terranes are
limited to areas underlain by carbonate rocks.

Figure 1. Map showing the distribution of carbonate rocks in Maryland. Those most associated with collapse sinkholes are the Hagerstown Valley (HV), the Frederick Valley (FV), and the Wakefield Valley (WV). To a lesser degree, collapse sinkholes are found in Green Spring Valley (gs), Worthington Valley (wo), and Long Green Valley (lg).

No single landform is common to all karst areas, but one characteristic of
all karst landscapes is disrupted surface drainage due to loss of surface water
to the subsurface. Two examples of karst landforms are caves and closed depressions,
known as sinkholes. Maryland has approximately 100 known caves, and many more
sinkholes. The carbonate rock areas of Maryland that exhibit some degree of
karst development are shown in Figure 1.

Potential environmental problems in karst terranes fall into two broad categories:
(1) groundwater pollution and (2) foundation engineering problems.
This fact sheet discusses three main categories of interrelated foundation
engineering problems: (a) differential compaction and settling due to
the irregular surface between soil and bedrock; (b) soil piping, which is a
type of subsurface erosion; and (c) collapse of the land surface into an underground
cavity--that is, collapse sinkholes.

Differential Compaction and Settling

Building the foundation for a house or other structure involves laying a footer,
usually pouring concrete into a level trench that outlines the dimensions of
the house, then erecting walls on that footer. Where there is a basement, a concrete
floor is generally placed on the bottom of the excavation within the confines
of the footer and basement walls. As the house is built, all of the weight, or
load, is carried by the outside walls and footer. In some karst terranes, that
can lead to problems.

In a type of karst known as cutter-and-pinnacle karst, the contact between
bedrock and soil overburden is very irregular (see Fig. 2 and 3 for example).
Water preferentially dissolves bedrock along some planar feature, such as bedding,
joints, or fractures, whichever is the easier path. Roughly vertical, solutionally
widened joints are called cutters, or grikes. Cutters are generally filled
with soil. The bedrock that remains between cutters may be reduced to relatively
narrow "ridges" of rock, called pinnacles, particularly where cutters
are closely spaced. Cutter-and-pinnacle karst (or simply "pinnacle karst" for
short) is common in many of the carbonate valleys in Maryland (Fig. 1).

The problem develops when a building foundation lies on cutters and pinnacles.
The weight of the building will compact the soil to some extent, and the building
will settle. That is normal, and does not pose a problem as long as the building
settles uniformly. However, in pinnacle karst, part of the foundation may be
supported by a bedrock pinnacle and part may be supported by a cutter (soil-filled).
The result can be differential settling of the building, which may produce
cracks in the walls, foundation, and floor (Fig. 2). This may compromise the
structural soundness of the bearing walls and, therefore, place the safety
of the whole structure in doubt.

Large buildings (schools, shopping centers, office buildings, etc.) commonly
have a detailed engineering design to avoid such potential problems, but private
homes are often built with little regard to such problems. Adequate site evaluation
prior to building is important; done properly, it can do much to prevent damage
to a home from differential settling.

Subsurface Erosion (Piping)

Piping is subsurface erosion of soil by percolating waters to produce pipe-like
conduits underground. Piping can affect materials ranging from clay-size particles
(less than 0.002 mm) to gravels (several centimeters), but is most common in
fine-grained soils such as fine sand, silt, and coarse clay. The resulting "pipes" are
commonly a few millimeters to a few centimeters in size, but can grow to a meter
or more in diameter. They may lie very close to the ground surface or extend
several meters below ground.

Figure 2.-- Cross-section sketch illustrating structural damage

Piping can become a problem in areas of cutter-and-pinnacle karst, as well
as in some non-karst areas. As shown in Figure 3, what begins as piping can
develop into cavities in the soil overburden. Piping tends to become accelerated
when the water table is lowered by over-pumping ground water, when the amount
of infiltrating water increases, or both. (The "water table" marks
the top of the zone of saturation, in which all pores and voids in bedrock
and soil are filled with water.)

What can cause increased volume of water that infiltrates the soil overburden?
Long periods of rainfall can be a factor, but man's activities also are significant.
Buildings with large roof areas, parking lots, streets and highways change
the runoff and infiltration characteristics of soil by decreasing widespread,
diffuse infiltration and channeling surface runoff to areas where more concentrated
infiltration can occur. Figure 3 shows how runoff can be concentrated in the
subsurface to create subsurface cavities. This is especially common in soil-filled
cutters.

One consequence of modification in runoff and infiltration can be foundation
problems similar to those mentioned in the previous section. Figure 3 shows
how a leaking storm drain or water main may lead to the pipe breaking and how
runoff from streets and houses may create subsurface cavities near building
foundations. This, it should be added, can occur in non-karst areas too. Because
the pavements and buildings act as supporting structures, the loss of soil
may not be apparent until a sizable cavity has developed. At some point, structural
support is lost. The result may be a relatively slow subsidence of the street
or the building, during which cracks will develop in basement walls and floors,
or the result may be a sudden collapse of the building or pavement. It should
be noted, also, that rerouting of storm runoff from rooftops, parking lots,
and streets can cause soil piping under adjacent properties.

Collapse Sinkholes

As used here, the term sinkhole refers exclusively to one type of closed depressions
in karst landscapes. One type of sinkhole is the collapse sinkhole, so named
because it forms suddenly when the land surface collapses into underground
voids, or cavities. Collapse sinkholes are often fairly circular with steeply
sloping sides. They can be so small as to be barely noticeable to 50 meters
or more in width and depth. Once formed, they can also grow larger.

In some karst terranes, collapse sinkholes form when the roof of a cave or cavern
collapses. Such is the case in some collapses in Florida (Sinclair, 1982). However,
most collapse sinkholes seem associated with cavities in the soil overlying the
carbonate rock. Some prefer the term cover collapse sinkhole to denote that collapse
occurs in cavities in the soil overburden, or cover, rather than in the carbonate
bedrock below. This is the general case for collapse sinkholes in areas of pinnacle
karst in Maryland.

Other types of sinkholes form slowly by the dissolving of carbonate rock at
or very near the surface. They tend to have gently sloping sides, and they
seldom pose a hazard by collapsing. Like collapse sinkholes, however, they
can pose environmental problems related to pollution, because they provide
a point where polluted surface runoff can directly flow into the ground water.

In the United States, according to one study, the states most impacted by
collapse sinkholes are Alabama, Florida, Georgia, Missouri, Pennsylvania, and
Tennessee (Newton, 1987).

In Maryland, collapse sinkholes occur mainly in four areas: the limestones
of the Hagerstown Valley in Washington County and the Frederick Valley in Frederick
County, marble in the Wakefield Valley in Carroll County and, to a lesser degree,
in marble valleys of Baltimore County (Fig. 1). Collapse sinkholes seem to
be most prevalent in the Frederick Valley and the Wakefield Valley.

Cavities of various sizes tend to develop in the soil overburden where infiltrating
surface waters erode the soil by piping and transport it downward through bedrock
cracks, or joints, that are themselves widened and enlarged by the dissolving
of the rock by the infiltrating water. This creates something like a plumbing
system through which the eroded soil overburden is carried.

Infiltrating CO2-charged water dissolves more and more carbonate rock over
long periods, sometimes enlarging the cracks to a meter or more in width. These
solutionally enlarged joints are most effective in transporting soil when they
are above the water table. A lowering of the water table, therefore, tends
to increase that effectiveness. In time, this process can produce a large cavity
in the soil overburden (Fig. 4).

Figure 4. Cross-section sketch showing the progressive development of a cavity in the soil overburden and eventual creation of a collapse sinkhole. Once the soil "bridge" over the cavity cannot support itself, collapse occurs (after Newton, 1987).

Moisture conditions in the soil overburden are important in that moisture
affects soil strength. Many factors can affect soil moisture--climate, soil
texture, soil mineralogy, evaporation, transpiration (the uptake of moisture
by vegetation), and sometimes depth to water table, to name a few. The interplay
among the factors is complex, but the net result is that "some" moisture
enhances soil strength; too much or too little diminish it.

For example, imagine a soil cavity exists a few meters below the surface.
Following a period of much rain, soil moisture content may become high enough
that it effectively reduces the strength of the soil, allowing collapse to
occur. In a few cases, the weight of the extra water from infiltrated rain
may even be enough to trigger a collapse. On the other hand, in a period of
drought, drying can reduce soil strength. Drying tends to cause shrinking,
which causes cracking, which in turn can lead to spalling of soil from the
roof of the cavity, eventually resulting in collapse. Thus, some soil moisture
is good because it increases the strength and reduces erodibility of the soil,
but too much or too little moisture generally reduces the strength of the soil
and makes collapse more likely.

The water table and soil moisture fluctuate naturally during the year in response
to patterns of precipitation, evaporation, and transpiration. Ground water
is not static, or stationary; it moves through pores and cracks in the bedrock
along a gradient, or "slope," from higher elevations to lower, eventually
discharging into streams. However, pumping a well or group of wells (residential,
commercial, municipal, or industrial wells) forms a cone-shaped depression
in the water table around the well(s). If a cone of depression becomes large
enough due to prolonged pumping, possible consequences include occurrence of
a collapse sinkhole, an increase in soil piping, loss of water in nearby wells,
dried up springs or streams, ground subsidence, or even reduction of support
beneath foundations of buildings. In cases such as these, one often hears the
terms, dewatering and overpumping.

Dewatering due to overpumping is not the only manmade cause of collapse sinkholes.
The same kinds of alterations to drainage discussed previously are often involved
too. A study of collapse sinkholes occurring over a nearly fifty-year period
in Missouri (Williams and Vineyard, 1976) showed that about half of collapse
sinkholes were natural and about half were man-induced. Altering drainage conditions
was found to be the chief manmade "cause" (Table 1). These observations
apply to Missouri only, but they do show that there are several possible triggering
mechanisms.

A natural collapse is the product of a process that can span many thousands
of years. Human activities (such as those listed in Table 1) or unusually wet
or dry weather can trigger collapse where the natural process has set the stage
for transport of soil from a growing void. However, correctly determining the
immediate cause, or trigger, of a collapse sinkhole is often very difficult.

Table 1.-- Frequency of collapse sinkholes in Missouri on
the basis
of "immediate" cause, as derived from records kept from 1930
through
the mid-1970s (after Williams and Vineyard, 1976).

Collapse
Sinkholes in Missouri

Number by natural causes

51

Number man-induced

46

Altered drainage

24

Water impoundments

10

Dewatering

7

Highway construction

3

Blasting

2

Accurately predicting collapse sinkholes is also very difficult. Conditions
must be such that collapse is imminent. In other words, any of a number of
contributing processes must have operated for some time. Often, there is no
warning that a cavity in the soil has developed until a collapse occurs.

In areas of known collapse sinkholes, frequent observations have sometimes
proven useful. Sinclair (1982) lists the following possible precursors of sinkhole
collapse:

Slumping or sagging. Tilting of fenceposts or other objects from the vertical.
Doors and windows that fail to open and close properly may be early warnings.

Structural failure. Cracks, however small, along mortar joints in walls
and in pavements may indicate subsidence.

Ponding. The ponding of rainfall may be the first indication of actual
land subsidence.

Vegetative stress. One of the earliest effects at an incipient sinkhole
is lowering of the water table. The lowered water table may result in visible
stress (e.g., wilting) to a small area of vegetation.

Turbidity in well water. Water sometimes becomes turbid during the early
stages of development of a nearby sinkhole.

It is important to realize that each of these things can occur for other reasons--and
in non-karst areas. However, if any of these observations are made in an area
of known sinkholes and if roads or buildings are potentially at risk, the property
owner should consider acquiring the services of an engineering geologist or
a foundations engineer. If you find a newly formed or forming collapse sinkhole,
please report it at once. • If on State highway right-of-way, call the
State Highway Administration at 410-321-3107 (or call the district office listed
in the blue pages of your telephone book).

If near an active quarry, call the Mining Program of the Maryland Department
of the Environment, 410-631-8055.

In other cases, contact the nearest office of the U.S. Department of Agriculture's
Natural Resources Conservation Service (also listed in the blue pages).

Direct general inquiries to the Maryland Geological Survey.

A final word of caution: Never climb into a collapse sinkhole, and never go
into any visible opening at the bottom of a sinkhole--especially if you are
alone. Always exercise caution when walking around sinkholes.

References

Newton, J. G., 1987, Development of sinkholes resulting from man's
activities in the eastern United States: US Geological Survey Circular
968, 54 p.

Prepared by James P. Reger
Permission is granted to reproduce this Fact Sheet so long as proper credit
is given to Maryland Geological Survey.
Compiled by the Maryland Geological
Survey, 2300 St. Paul Street, Baltimore, MD 21218
This electronic version of "Fact Sheet No.11 " was prepared by R.D.
Conkwright, Division of Coastal and Estuarine Geology, Maryland Geological
Survey.